It is frequently desirable to recover metal substrates which have been electroplated with nickel or iron nickel alloys, especially when the plating is defective. Similarly, it becomes desirable from time to time to strip electroplating from clamping devices and frames used in the electroplating of base metals, e.g., steel. The recovery of such plated metal substrates or stripping of clamps, frames and other plating bath equipment has been hampered by the inability to obtain adequate stripping without excessive etching of the metal being stripped.
U.S. Pat. No. 3,912,603 discloses a method for stripping electroplating from steel which comprises the use of alkali metal or ammonium iodides, preferably the iodides which are difficult to solubilize, such as copper-I-iodide. The stripping bath contains nitric acid or its salts, e.g., alkali metal, ammonium or organic amine salts. The stripping operation is carried out at a pH of about 6.5 to 7.5.
U.S. Pat. No. 4,048,006 discloses a method for stripping electroplated nickel-iron alloys from a metal substrate using a nitro substituted organic compound in combination with an aliphatic carboxylic acid or salts thereof which contain at least one substituent which is --NH.sub.2 or quaternay amine salt, --OH or --SH and at least one organic amine or polyamine or substituted analogs thereof. The process is advantageously carried out at elevated temperatures, e.g., 60.degree. C. to 90.degree. C. at a pH below 7. The process can be carried out with nickel-iron alloy deposits which contain minor amounts of impurities such as zinc, cadmiun, lead, etc.
U.S. Pat. No. 4,261,804 discloses a method for stripping nickel alloys from ferrous substrates which utilize specific voltage conditions to strip the nickel alloys from the substrate without damage to the substrate metal.
U.S. Pat. No. 4,052,451 discloses a composition and process for the selected stripping of nickel-iron alloys from ferrous substrates which comprises contacting the metal surface with an aqueous solution containing at least one nitro substituted compound, at least one organic amine or polyamine and substituted analogs thereof, and at least one phosphorous compound which is a phosphorous oxo acid or organic phosphorous oxo acid or salts thereof or alkyl phosphonate substituted amines.
U.S. Pat. No. 4,233,124, and a related patent, U.S. Pat. No. 4,264,420, disclose an electrolytic bath composition and process for stripping metal deposits from a different substrate metal. The composition comprises an aqueous solution containing activating halogen compounds, an amine, nitro and/or nitrate stripping components. Additionally, an inhibiting agent to inhibit attack on the substrate metal comprising glucoheptonic acid, malic acid and mixtures thereof as well as the Groups IA, IIA and ammonium salts is included in the composition. The halogen containing compounds are preferably bromine compounds which liberate bromine ions to accelerate the stripping action. A carboxylic acid buffering agent is used to control pH.
In an article appearing in Transactions of the Institute of Metal Finishing, 1981, Vol. 59, pp. 110-112, entitled "Electrolytic Stripping of Nickel and Other Metal Coatings From Steel Substrates" by J. O'Grady, a method for stripping nickel alloys from steel substrates is disclosed wherein the stripping agent is ammonium nitrate and carboxylic acids are used to inhibit attack of the substrate metal. The process is carried out at a pH of about 4.5 to about 6.5.
Invariably, prior art techniques have utilized direct current in the stripping process which is consistent with the manner in which the electroplating was initially deposited. Prior art methods of stripping nickel and nickel-iron alloys from a substrate metal have generally resulted in excessive etching of the substrate metal.